Gregers asks if Hialmar leaves the work of the studio to his wife. Hialmar is surprised that he has yet to hear of his "invention." Hialmar has dedicated himself to photography to produce a great invention and redeem the family's name. He cannot, however, explain the invention.
Gesturing toward the pistol, Hialmar reveals that Old Ekdal almost killed himself upon his prison sentence, but his courage failed him. Hialmar too put the pistol to his breast when his father went to jail. Ekdal's departure left existence "at a standstill—as if under an eclipse."
Gregers wonders if Hialmar's time in the garret does not distract from this supposed invention. Hialmar counters that the garret fills the time while he awaits his inspiration. Gregers remarks that Hialmar has something of the wild duck in him. He has dived down into the undergrowth and strayed into a "poisonous marsh." A disease has overtaken him, and he is sure to die. His contentment is an effect of the marsh poison. Hialmar moves to silence him. In his house, no one speaks to him of unpleasant things.
Gina and Hedvig bring lunch, and Relling and Molvik appear. Relling is an old foe of Gregers from the Hoidal works. Relling jests that he is lucky to have both priest and doctor under the same roof; Gregers replies that he might need them, as there were thirteen at the table last night. They begin lunch. Relling relates that Molvik was drunk last night. Molvik is apparently prone to demonic possessions. He disparagingly asks if Gregers ever managed to collect on that claim he was presenting throughout the works—the "claim of the ideal." The group toasts Ekdal and Relling cheers Hialmar and his family. Apparently it is Hedvig's birthday tomorrow. Saddened by the modesty of tomorrow's affair, Hialmar promises his daughter great prosperity upon the invention's completion.
Relling comments fondly on the "happy fondly circle." Gregers retorts that he does not thrive in "marsh vapors." Gina assures him that she airs the house everyday. Relling wonders if Gregers himself has brought the "taint" into the house with his "claim of the ideal" and threatens to throw him out. It is hardly appropriate for him to talk of vapors and taints considering the mess he made with his stove.
A knock is heard at the door, and Hakon Werle appears. He takes his son aside for a private talk. Gregers announces his mission to open Hialmar's eyes and show him his position as it really is. Stricken with guilt over his failure to save Ekdal from his father, he can now rescue Hialmar from the falsehoods that are bringing him to ruin. Werle warns that Gregers hardly does his friend a kindness. His conscience has been sickly from childhood; it is an inheritance from his mother.